Exactly how common is sleep paralysis? This is a difficult question to answer.
A variety of studies have looked into the percentage of people who experience the phenomenon, reporting estimates from 5% to 65%. There are several reasons why such widely fluctuating results have been found. Firstly, the number of individuals looked at in each study differs. Many studies have samples as low as 60 participants, which is unlikely to give a representative sample. Furthermore, studies have been confounded by the inclusion of particularly at-risk groups. Cultural differences are also likely to play a role. For instance, studies have found a higher prevalence of sleep paralysis in African American samples compared with white Americans, with some suggesting that social factors may be behind this. It has been suggested that groups with a high awareness of sleep paralysis and/or strong cultural beliefs surrounding the experience report higher rates.
In their 2011 paper Prevalence of Sleep Paralysis, Sharpless and Barber combined more than 30 studies on sleep paralysis prevalence from a variety of cultures and groups, giving them a sample size of over 36,000. They concluded that approximately 8% of people experience sleep paralysis, rising to around 28% in high risk groups (those that have a disrupted sleep pattern) and up to 34% in those suffering from psychiatric disorders such as anxiety and depression.